The embodiments relate to a magnetic resonance (MR) device with at least one distribution network for distributing an electrical input signal to a number of feeding points of an MR antenna. The embodiments are applicable, for example, to the feeding of a body coil of an MR device, especially a so-called birdcage antenna.
For MR imaging, at least one so-called B1 field is created by an MR transmit antenna at the location of a body, especially a patient, to be examined. In this case the MR antenna frequently has a number of antenna subsystems, which are able to be activated separately, in order to create a vertical or a horizontal field component for example, that together bring about a circular polarization of the B1 field.
At higher basic field strengths B0, as from approximately two Tesla, influencing of the MR transmit antenna by the patient can result in disturbances in the creation of the B1 fields. The influencing acts through a different loading of the antenna subsystems for example, through which the vertical and horizontal field components are attenuated differently. The influencing can also act on an antenna impedance, for example, because of a complex reflection factor and/or a coupling. Both lead to the fields ultimately created not corresponding to the desired field, for example in relation to an ideal circular-polarized excitation. In addition, there are also direct interactions between the patient and the electromagnetic fields created by the antenna. In the final analysis, both interactions lead to inhomogeneities occurring in the transmit and receive profiles. The inhomogeneities lead in part to marked variations in intensity in the MR images established therefrom.
One approach to solving these problems is to use multi-channel MR transmit systems. In such systems, the MR transmit antenna has N independent antenna elements that are activated by N independent RF transmitters. The activation can be set by suitable prescan methods so that at least one homogeneous field distribution is achieved during the excitation of the spins.
The simplest form of embodiment of a multi-channel MR transmit system used in practice is a two-channel transmit system in which the MR transmit antenna is able to be activated at two feeding points offset by an angle of 90° to each other by RF transmit pulses able to be created independently. However, the two-channel MR transmit system possesses too few degrees of freedom to be able to satisfactorily resolve the problems outlined above. There are MR transmit systems with more than two transmit channels, primarily for even higher basic field strengths B0 as from approximately seven Tesla, but, because of their complexity and costs, these are not yet being used in clinical systems.
A simple variant for exerting an influence on the homogeneity of the field components is the use of a transmit antenna with a four-port feed, as is shown in FIG. 1. As a rule, this involves a circular-polarized MR antenna (e.g. a so-called birdcage antenna BC), in which in each case two feeding points F1-A and F1-B or F2-A and F2-B lying 180° opposite to one another are connected to a common feed cable (and thus belong to the same channel), wherein a phase offset of 180° exists between the fed-in signals of a channel.
To this end, FIG. 2 shows a schematic layout of an MR device MRT with an MR transmit antenna in the form of a birdcage antenna BC with the feeding points F1-A, F1-B, F2-A and F2-B. The birdcage antenna BC has two distribution networks N1 and N2 or is connected to such networks. The distribution networks N1 and N2 are configured to divide up incoming input signals Tx-Ch1 and Tx-Ch2 of the two channels Ch1 or Ch2 (e.g. excitation pulses created by an RF transmitter) into part signals Tx-Ch1-A and Tx-Ch1-B or Tx-Ch2-A and Tx-Ch2-B in each case and to distribute the divided part signals to the feeding points F1-A and F1-B or F2-A and F2-B. Corresponding signal inputs I1 and I2 of the distribution networks N1 or N2 can be connected to respective transmitters (not shown in the figure), which in particular can create the input signals Tx-Ch1 or Tx-Ch2 independently.
Expressed more precisely, an input signal Tx-Ch1 or Tx-Ch2 fed into a signal input I1 or I2 can be divided up by the respective distribution network N1 or N2 into the respective two part signals Tx-Ch1-A and Tx-Ch1-B or Tx-Ch2-A and Tx-Ch2-B, which are then present at corresponding signal outputs O1-A and O1-B or O2-A and O2-B of the two distribution networks N1 or N2. The two part signals Tx-Ch1-A and Tx-Ch1-B or Tx-Ch2-A and Tx-Ch2-B of a respective input signal Tx-Ch1 or Tx-Ch2 or channel Ch1 or Ch2 are phase-shifted at the signal outputs O1-A and O1-B or O2-A and O2-B as well as at the feeding points F1-A and F1-B or F2-A and F2-B by phase offset Δφ=180°.
For even distribution of the power of an MR transmitter to the assigned feeding points F1-A and F1-B or F2-A and F2-B, true power dividers (e.g. Wilkinson dividers, hybrids etc.) can be used together with phase shifters. To this end, FIG. 3 shows an equivalent circuit diagram of a conventional distribution network Nx with x=1 or 2. The distribution network Nx has four nodes K1 to K4, wherein an inductance or coil L is connected in each case between the nodes K1 and K2 and also K3 and K4, while a capacitance or capacitor C is connected in each case between the nodes K1 and K3 or K2 and K4. The first node K1 is connected to the signal input Ix, the second node K2 via a first discrete phase-shifting component PS-A to the first signal output Ox-A, the third node K3 via a second discrete phase-shifting component PS-B to the second signal output Ox-B, and the fourth node K4 to ground GND. The first phase-shifting element PS-A shifts an electrical signal coming from the second node K2 by 90° or Lambda/4. The second phase-shifting element PS-B shifts an electrical signal coming from the third node K3 by −90° or by 270° or −Lambda/4 or 3/4 Lambda. The signals present at the first signal output Ox-A and at the second signal output Ox-B are thus phase-shifted in relation to one another by 180°, but especially have the same amplitude or power. The disadvantage here is the high demand for discrete components such as coils, capacitors, resistors etc. together with the necessary balancing of them.